Diff for "BashFAQ/042"

How can I find out if a process is still running?

The kill command is used to send signals to a running process. As a convenience function, the signal "0", which does not exist, can be used to find out if a process is still running:

# Bourne
myprog &          # Start program in the background
daemonpid=$!      # ...and save its process id

while sleep 60
do
    if kill -0 $daemonpid       # Is the process still alive?
    then
        echo >&2 "OK - process is still running"
    else
        echo >&2 "ERROR - process $daemonpid is no longer running!"
        break
    fi
done

NOTE: Anything you do that relies on PIDs to identify a process is inherently flawed. If a process dies, the meaning of its PID is UNDEFINED. Another process started afterward may take the same PID as the dead process. That would make the previous example think that the process is still alive (its PID exists!) even though it is dead and gone. It is for this reason that nobody other than the parent of a process should try to manage the process. Read ProcessManagement.

This is one of those questions that usually masks a much deeper issue. It's rare that someone wants to know whether a process is still running simply to display a red or green light to an operator.

More often, there's some ulterior motive, such as the desire to ensure that some daemon which is known to crash frequently is still running. If this is the case, the best course of action is to fix the program or its configuration so that it stops crashing. If you can't do that, then just restart it when it dies:

# POSIX
while true
do
  myprog && break
  sleep 1
done

This piece of code will restart myprog if it terminates with an exit code other than 0 (indicating something went wrong). If the exit code is 0 (successfully shut down) the loop ends. (If your process is crashing but also returning exit status 0, then adjust the code accordingly.) Note that myprog must run in the foreground. If it automatically "daemonizes" itself, you are screwed.

For a much better discussion of these issues, see ProcessManagement or FAQ #33.

CategoryShell